TRACING THERMOHALINE PROPERTIES AND PRODUCTIVITY OF SHELF-WATER MASSES USING THE STABLE ISOTOPIC COMPOSITION OF BENTHIC FORAMINIFERA

Abstract

We explored the applicability of stable isotopic ratios of nine foraminiferal species, four epifaunal (Pseudononion atlanticum, Cibicides fletcheri, Hoeglundina elegans, Hanzawaia boueana) and five infaunal (Angulogerina angulosa, Uvigerina peregrina, Buccella peruviana, Cassidulina subglobosa, Bulimina marginata), as environmental tracers on the southwest Atlantic continental shelf from samples collected along the Argentinean-Uruguayan-Brazilian Atlantic coast during austral winter 2003 and summer 2004. Linear regressions show that the stable isotopic composition of living foraminifera is associated with geographical and environmental variables (latitude, water depth, temperature, salinity, and dissolved silica). Multiple linear regressions show that the relevant variables in this study are latitude, salinity, and biogenic silica. There is a general trend of higher δ 18 O and lower δ 13 C values with increasing latitude, reflecting the influence of cooler water masses toward the south. Specifically, the δ 18 O data from U. peregrina, H. elegans, and P. atlanticum follow meridional temperature gradients with lower δ 18 O values in relatively warm Subtropical Shelf Water at the northernmost sites (27°S) and higher values in colder Sub-Antarctic Shelf Water at the southern sites (to 37°S). Angulogerina angulosa and P. atlanticum δ 18 O values correlate more strongly with salinity than temperature. Positive correlation of δ 13 C and water depth observed for some infaunal species indicates greater input of organic matter to the deeper-water benthos. In summer samples, however, this correlation between δ 13 C and water depth weakens, reflecting a less stratified water mass associated with upwelling. Results from this study support the application of these foraminifera to reconstructions of paleoenvironmental changes from sediment cores.